Traditionally, when author wants some text to be set in italics, he will underline it (either with straight or wiggly line) to mark it for the typesetter. This is used whenever the italic text is not available or hard to make -- when using a typewriter or when handwriting.

Resources:

• http://www.wonderfulwritingskillsunhandbook.com/HTML/Information/Punctuation/italicsunderline.html
• Wikipedia:Emphasis_(typography)
• Meatball:CharacterFormattingRules
• C2:RealMenDoNotUnderline